Polybutylene terephthalate (PBT) reinforced with thermally stable reinforcing fibers such as glass fibers is well known as a molding resin and is described in numerous patents and publications including for instance U.S. Pat. No. 3,814,725, U.S. Pat. No. 3,814,786 and U.S. Pat. No. 3,624,024. Fiber reinforcement generally improves the tensile strength, flexural strength, flexural modulus and heat deflection temperature of the molding composition. However, moldings, especially injection moldings of large fiber reinforced articles of PBT, tend to display distortion or warping while some other resins of otherwise less desirable properties do not present such problems. It is believed that strains resulting from the different rates at which resin and glass contract during the cooling of molded articles are responsible for such warping. The warping is thus believed due to the presence of the very reinforcing fibers which contribute to the enhanced physical characteristics of the finished product. It is known that addition of mica to fiber glass reinforced PBT reduces warping. Unfortunately, the mica also greatly reduces impact strength.